Don't panic, but anyone planning on using GPS in the southeastern US for the next month or so will likely want to make sure they have a fallback option. That's according to a flight advisory just issued by the FAA, which warns pilots that their GPS signal "may be unreliable or unavailable" due to "GPS tests" that the Department of Defense will be conducting between January 20th and February 22nd. Details are fairly light beyond that, but the FAA does note that when the "tests" occur they will be active for 45 minutes, and be followed by 15 minutes of off time -- additional notices to pilots will apparently be issued at least 24 hours prior to any test. Of course, while this particular advisory is directed at pilots, it will presumably will also affect all other GPS devices, as the FAA doesn't mention any GPS issues specific to aviation. The real question, however, is just what those mysterious "GPS tests" are -- if they're, in fact, GPS tests at all...